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A database concept used to represent missing, unknown or inapplicable data.

42
votes
I have seen database interfaces (e.g. framework libraries) that return 'null' as a string for null columns. I believe there was a flag that would turn this on or off for debugging. This flag … allows developers to easily determine if the empty field was a result of a null value or an empty value. This is a bad setting, especially in production, and would explain the issues explained in the …
answered Apr 18 '16 by BillThor
1
vote
Don't replace people with a separate table for each person. You may want child tables for types that carry type specific data. These will have the same primary key as the people table, but only exis …
answered Jul 29 '17 by BillThor
1
vote
Oracle treats NULL as an unknown value. Ask yourself if this equality works. (unknown amount in George's wallet) = (unknown amount in Harry's wallet) or otherwise stated NULL = NULL The … answer is clearly maybe, which is neither true nor false. EDIT: In response to the comments I will add a little clarification to NULL as I have seen it used. While NULL really means "not set", in the …
answered Aug 3 '11 by BillThor
2
votes
I beg to disagree with this author. The results are expected, but may not be the desired values. Null values don't work well with logical comparisons as the value is unknown, as is the result of a … comparison. If you are using nullable columns as query criteria, you do need to handle them appropriately. These are some of method I have used: WHERE (nullable = ? OR nullable IS NULL) WHERE …
answered Apr 3 '14 by BillThor
2
votes
While I do use NULL columns, there is overhead. The Oracle documentation you retrieved this short list should explain if you read further. There are case where NULL indicates issues with data … types and/or just having the column. Consider PHONE_NUMBER NUMBER(15): This will likely have a formatted phone number column, and may be null for numbers like 1-800-CALL-NUM. Other columns may be …
answered Jul 17 '16 by BillThor